Apparatus for shrouding a pouring stream and molten casting surface with a protective gas

ABSTRACT

A METHOD AND APPARATUS FOR SHROUDING A STREAM OF MOLTEN METAL AND THE CASTING MENISCUS WITH A PROTECTIVE GAS AS THE METAL IS POURED FROM A NOZZLE INTO A MOLD. APPARATUS INCLDUES AN ENCLOSURE AT THE TOP OF THE MOLD AND MEANS FOR INJECTING GAS INTO THE ENCLOSURE AT A LOCATION REMOTE FROM WHERE THE STREM ENTERS. ENCLOSURE HAS A   RESTRICTED OPENING AT THE TOP THROUGH WHICH STRAM ENTERS AND GAS ESCAPES TO ENVELOP STREAM.

March 23, 1971 R. E. LYMAN 3,572,422

APPARATUS FOR SHROUDING' A PQURING STREAM AND MOLTEN CASTING SURFACE WITH A PROTECTIVE GAS Filed Dec. 10, 1968 11' F P A 1 5 INVENTOR.

RICHARD E. LYMAN A Harny United States Patent APPARATUS FOR SHROUDING A POURING STREAM AND MOLTEN CASTING SURFACE WITH A PROTECTIVE GAS Richard E. Lyman, Homewood, [1]., assignor to United States Steel Corporation Filed Dec. 10, 1968, Ser. No. 782,578 Int. Cl. B22d 11/00 US. Cl. 164-273 3 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for shrouding a stream of molten metal and the casting meniscus with a protective gas as the metal is poured from a nozzle into a mold. Apparatus includes an enclosure at the top of the mold and means for injecting gas into the enclosure at a location remote from where the stream enters. Enclosure has a restricted opening at the top through which stream enters and gas escapes to envelop stream.

This invention relates to an improved method and apparatus for shrouding a molten metal pouring stream and the casting meniscus with the protective gas.

It is known to protect a molten metal pouring stream against oxidation by shrouding it with inert or reducing gas as it passes from a ladle or tundish nozzle into a mold, as shown for example in 'French Pat. No. 1,223,358. However, previous shrouding arrangements have not afforded maximum protection to the metal. Desirably the nozzle outlet is accessible to permit removal of freezing drip points with a tool or torch flame. Otherwise such drip points may grow to an extent that they cause serious spraying and misdirection of the pouring stream, or the metal may actually become welded to the top of the mold. To make the outlet accessible it is necessary to leave space between the outlet and the shrouding apparatus, and the portion of the stream within this space is protected only by gas escaping from the latter. Previous arrangements, such as that shown in the French patent, lack any provision for maintaining positive pressure in the shrouding apparatus. Hence air may be drawn into the shrouding apparatus, and there is no assurance that gas escaping therefrom properly envelops the pouring stream.

An object of my invention is to provide an improved shrouding method and apparatus which leaves the nozzle outlet accessible, yet assures that the protective gas properly envelops both the pouring stream and casting meniscus, thereby effectively preventing oxidation of the metal.

A more specific object is to provide an improved shrouding method and apparatus in which the pouring stream passes through an enlarged chamber with a restricted opening, and a protective gas is injected to this chamber, where it is maintained under positive pressure and escapes through said opening to envelop the pouring stream, as well as covering the casting meniscus.

In the drawing:

FIG. 1 is a vertical sectional view of my improved shrouding apparatus positioned between a nozzle and mold; and

FIG. 2 is a horizontal section on line II-II of FIG. 1.

FIG. 1 shows portions of a bottom-pour vessel 10, commonly a ladle or tundish, and a mold 12, which may be part of a continuous-casting machine or an ingot mold. Vessel has a nozzle 13 through which a pouring stream of molten metal S discharges into mold 13, where it forms a pool P having a meniscus at its upper surface. Usually the metal has been degassed in a vacuum apparatus, as known in the art. The vessel may be equipped with any suitable closure means (not shown) for controlling dis- 3,572,422 Patented Mar. 23, 1971 charge or metal through nozzle 13. The foregoing parts and their operation are conventional, and hence are not shown or described in more detail.

My improved shrouding apparatus includes a supporting plate 14 and an open-bottom box 15 resting on said plate. Conveniently the plate may be fixed permanently to the top of mold 12. Preferably box 15 is square in plan and is formed of sections 15a and 15b separable along the transverse center line 16 of the box (FIG. 2). The sectional construction enables me to remove the box while metal is being poured without interrupting the operation. The top of box 15 is spaced below the outlet of nozzle 13 by a distance x, which equals about /2 to 4 times the stream diameter d. The top has a restricted opening 17 through which stream S enters the box and mold. The cross-sectional area of the box is large in relation to that of the stream, and the diameter y of its opening 17 is about 2 to 4 times the stream diameter d. The area of opening 17 should be less than 25 percent of the cross-sectional area of a mold having a cross-section greater than about square inches, and less than 50 percent if the mold has a smaller cross section. Preferably the top has a viewing port 18 and a transparent window 19 covering the port. Plate 14 contains a gas inlet tube 20 and a sampling tube 21. The inlet tube has a downturned segment 22 at its discharge end directed toward the surface of pool P in mold 12.

According to my shrouding method, I inject a suitable protective gas, such as argon, into the enclosure formed by mold 12 and box 15 via the inlet tube 20. The gas discharges downwardly into mold 12 through the downturned segment 22 at a location remote from the opening 17. Thus the gas covers the meniscus at the top of pool P. The gas is under sufficient pressure, as hereinafter demonstrated, that it builds up a positive pressure with the enclosure and prevents air from entering. The gas escapes from the box through the opening 17, flowing upwardly around stream S as indicated by arrows 23. The gas also forms a concentric eddy current zone 24 adjacent stream S induced by a moving boundary layer at the stream surface.

In more detail, I develop a dynamic pressure within box 15 according to the relation:

P =density of the injected gas at standard temperature and pressure Q =fiow of the gas at standard temperature and pressure A=cross-sectional area of opening 17 T=absolute temperature of the gas discharge T =standard absolute temperature (528 R.)

g=acceleration of gravity in which If, for example,

P =0.10 lb./ft.

Az o ft. (for 3-in ID) T: l700-i-460=2160 R. Then P=0.282 lb./ft.

culations are conservative, since the actual flow area does not include the area occupied by the pouring stream S and the concentric eddy zone 24. The actual gas velocity and corresponding dynamic pressure are greater than those calculated.

The injected gas fiows through the mold and box and continuously purges the space within. Thus it provides atmosphere control beyond mere exclusion of air. The residence time of the injected gas is long enough that absorption of gases dissolved in the molten metal (by virtue of low partial pressure) is effective. If the injected gas is reactive (e.g. reducing), the residence time provides reactive efficiency.

I claim:

1. The combination, with a pouring vessel having a nozzle for discharging a stream of molten metal, and a mold spaced below said nozzle for receiving the metal, of a shrouding apparatus comprising:

means forming an enclosure at the top of said mold spaced below said nozzle and including a plate fixed to the top of said mold and an open-bottom box resting on said plate and having separable sections enabling it to be removed without interrupting a pouring operation;

said enclosure having a cross-sectional area which is large in relation to said stream the top of said enclosure having a restricted opening through which said stream enters an inlet tube extending through said plate and having 4 a downturned segment directed into said mold for injecting a protective gas into said-enclosure at a location remote from said opening and maintaining the gas under a positive pressure said opening forming an escape route for the gas to enable it to envelop the stream; and

a sampling tube extending through said plate.

2. A combination as defined in claim 1 in which said opening is of a diameter 2 to 4 times the stream diameter.

3. A combination as defined in claim 1 in which said box has a viewing port and a transparent window covering said port.

References Cited UNITED STATES PATENTS 204,307 5/1878 Duff l64259 2,023,957 12/1935 Hewgill l6466 2,060,133 11/1936 Summey 1 64259X 3,439,735 4/1969 Holmes l64259 3,099,052 7/1963 Suzuki et a1. l6466 3,467,167 9/1969 Mahin l6466 J. SPENCER OVERHOLSER, Primary Examiner V. K. RISING, Assistant Examiner US. Cl. X.R. 164-259, 66 

